Analog circuits typically make extensive use of voltage and current references. Such references are DC quantities that exhibit little dependence on power supply and fabrication process parameters, while also demonstrating a well-defined (or preferably no) dependence on temperature. Perhaps the most commonly implemented reference circuit is the bandgap reference circuit.
As is known, bandgap reference voltage circuits provide a substantially constant output reference voltage over a temperature range. To accomplish this, bandgap references provide temperature compensation so that the output reference voltage does not vary with temperature. Generally, the output reference voltage is a function of the base-to-emitter voltage (Vbe) of one bipolar transistor and the difference between the base-to-emitter voltages (ΔVbe) of a pair of bipolar transistors having different associated current densities. The value of the temperature independent reference voltage is generally adjusted by scaling ΔVbe. This arrangement provides the desired temperature compensation since Vbe of a bipolar transistor has a negative temperature coefficient while ΔVbe of a pair of bipolar transistors has a positive temperature coefficient. Thus, the temperature variations of the Vbe and the ΔVbe terms establishing the reference voltage can be made to cancel, thereby providing an output reference voltage that is essentially constant with respect to temperature.
Many conventional bandgap reference designs exist for producing stable reference voltages while driving relatively small load currents, e.g., 100 μA. However, some applications require reference voltages that can drive larger currents, e.g., currents on the order of 400 μA or greater. As the output current of a bandgap reference circuit increases, certain performance characteristics of the circuit become more difficult to control and/or the those performance characteristics become more important to overall circuit operation. For example, when driving large load currents it can be difficult to maintain as small a reduction in reference voltage (sometimes called “droop”) as possible. Moreover, maintaining low noise output can become more complicated with higher output current bandgap references. For example, one common noise reduction technique is to couple the output of the bandgap reference to a relatively large capacitance. With a Class A output stage, the large capacitor is the compensation capacitor and the dominant pole in the feedback loop, but the drive capability of this stage is very limited. With a Class AB output stage, the output is usually not the dominant pole and the large capacitor can cause the output and hence the feedback path to have too much phase shift and cause oscillations. Also, because the Class AB output is a lower impedance, the noise reduction is limited. Unfortunately, driving large capacitive loads can cause an additional pole in the feedback response of the amplifier used in conjunction with the reference devices. In addition, bandgap architectures frequently have more than one stable operating point. An additional stable operating point is often at 0 V and thus a “start-up” circuit must be added to avoid this undesired operating point. Still another concern is the ability to maintain relatively high Power Supply Rejection (PSR) for the bandgap reference.
One approach to the problem of producing high output current bandgap references is by using a bipolar technology, because of the higher device gain and lower device impedance for a given current. Unfortunately, such an approach is frequently not possible on standard CMOS technology processes due to non-existent or very poor bipolar transistors.
Accordingly, it is desirable to have bandgap reference circuit implementations that provide high output current yet still have relatively low noise output, have low voltage droop, have suitable levels of power supply rejection, and avoid settling into an intermediate or undesirable state.